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One of the most attractive aspects of a tree is the shade it casts. Before planting a tree in your yard, however, it is important to calculate the shade it will provide. Knowing the full height and spread of the tree you wish to plant, along with its growth rate, lets you project how tree shade will affect your house, flower beds or vegetable garden. Calculate shade potential when choosing, planting or maintaining a tree.

Assembling Tree Information

The maple sapling standing in your new back yard will, in time, grow large enough to create a cooling pool of shade. Its growth rate, mature height and possible mature branch spread, or canopy, are critical to whether your patio becomes a cool oasis or your springs are spent scrubbing mildew off your siding. Research the mature size of your tree before making a final choice. Your local environmental center or botanical garden can provide information on how large the tree variety you like tends to grow in your area, along with possible local growth rates. According to the Arbor Day Foundation, a slow growth rate is one of no more than 12 inches a year; a moderate rate is 13 to 24 inches a year; and a fast rate is 25 or more inches.

Choosing a Location

When calculating shade potential, results depend heavily on where your tree sits in the yard in relation to both your house and the sun. Trees planted south of your house will cast more shade over it than trees planted to the north. Choose a southeastern location to block morning sun, and southwestern to block afternoon sun. Remember that in the northern hemisphere shadows cast by a tree will be longer in winter and shorter in summer because of the changing seasonal tilt of the earth. You can learn a lot about shadow-length in your yard by observing seasonal changes in the shadow cast by your house. The difference in shade is determined by whether a tree is deciduous or evergreen; the dense branches of many evergreens offer a buffer against wind as well as light, while bare deciduous branches let you take advantage of the warming potential of winter sun.

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Estimating Shade

While calculating the exact path and dimensions of a tree's shade requires knowing the sun's position at any given time of the year, some general guidelines will help you determine where shade will fall in your yard. A mature tree 50 feet in height with a 30-foot canopy will cast a shadow the size of the canopy when the sun is directly over the tree. According to the University of Missouri, in summer, that same tree will cast a shadow approximately equal to its height at 3 or 4 p.m. In winter, because the sun is lower on the horizon, that same shadow will measure 120 feet -- more than twice the height of the tree -- at 3 p.m. Between the summer solstice, the longest day of the year, and the winter solstice, the shortest, the sun's height from the horizon can shift up to 50 degrees, notes the University of Missouri. Wherever you live in the northern hemisphere, you can expect winter shadows to be longer than summer ones.

Calculating Tree Height

If you are trying to assess the shade cast by an existing tree, you can determine the height using a hand level, or clinometer. If you don't have one, the Tennessee Department of Agriculture offers instructions for creating a basic guide: Measure the distance from your eye, more safely your cheekbone, to the point on your hand where the thumb and forefinger meet. Place a stick along that length and grasp it where it rests at the base of your thumb, and then hold the stick up vertically. If you face the tree and back away until it appears the same length as the stick with the base of the tree aligned with the top of your fist, the distance between you and the tree, when measured, will be the same as the height of the tree.

Other Planning Help

Increasingly, one of the most compelling reasons to plant a shade tree is to use its cooling capacities to lower utility costs. Estimates of savings range from 10 to 50 percent of home energy costs. Exact amounts will depend on tree height, shade density, home insulation and exposure and overall climate. A growing number of utility companies offer online shade-savings calculators or other estimating tools to help customers determine the best tree choices for their property. This can be useful whether you want to plant, prune or remove a tree.

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About the Author

Janet Beal has written for various websites, covering a variety of topics, including gardening, home, child development and cultural issues. Her work has appeared on early childhood education and consumer education websites. She has a Bachelor of Arts in English from Harvard University and a Master of Science in early childhood education from the College of New Rochelle.